Fitch last week reaffirmed Turkey's sovereign rating at "BB" with a negative outlook, but Parker said it could be downgraded if "existing weaknesses are aggravated, especially given the volatility in the Turkish economy and currency".

Turkey's main opposition party on Wednesday appealed to Turkey's election authority to cancel local poll results in all 39 districts in Istanbul.

It was the first time the opposition had taken control of Istanbul in 25 years and was particularly sensitive for Erdogan, himself a former mayor of the 15-million strong metropolis.

Former Turkish Prime Minister Binali Yildirim will again be the ruling AK Party's candidate in next month's re-run of the Istanbul mayoral election, Erdogan told reporters on Tuesday.

The AK Party said the decision of the Supreme Election Council was not aimed at safeguarding any party's interests.

The European Union requested that Turkey makes the justification for its decision publically available and to invite worldwide observers to monitor the rerun.

It was unclear how the CHP and its supporters would respond to a re-run given suspicions over the YSK's political independence from the AKP, which in recent years has centralized power in the presidency away from other institutions. Piri of the European Parliament also said pressure was applied to re-do the elections. "This system that overrules the will of the people and disregards the law is neither democratic, nor legitimate". On Saturday Erdogan said "it's clear" the vote was marred by controversy and urged the YSK to "clear its name" with a re-run.

Under the decision, Ekrem Imamoglu, a member of the CHP whose election as Istanbul mayor had been certified, had his certification revoked, with an interim mayor filling the post until the June 23 polls.

"Shame on the YSK".

In a series of tweets published on Tuesday evening, Davutoglu said events after the Istanbul election and the decision to annul it had harmed the "basic values" of Turkey's democracy - that voters had the final word at the ballot box.